The present invention generally relates to the field of structures for enhancing the heating, browning, and crisping of food products in microwave ovens. More particularly, the present invention pertains to microwaveable structures that have patterned conductive formations of a relatively large thickness that can be selectively modified to substantially absorb, reflect, and/or focus microwave radiation. The present invention further pertains to susceptor underlays that incorporate patterned conductive films for controlling temperature gradients within microwave susceptors.
In the following description reference is made to certain structures and methods. However, such references are not to be construed as an admission of prior art. Applicants reserve the right to dispute that such structures and methods qualify as prior art against the present invention.
Microwave susceptors are conductive structures that undergo heating when exposed to microwave radiation and are commonly employed in microwave food packaging to tailor the heating, crisping, and browning of microwave food products. A typical susceptor is a laminated structure comprised by a thin, microwave-absorbing layer disposed between a polymer barrier layer and a structural backing layer. Thin films of aluminum are most commonly used. Such a susceptor is typically formed by depositing a thin metallic film onto a polymer film substrate. The metallized polymer film is then often laminated to the structural backing layer. The laminate may then be used to form packaging for food products.
When exposed to microwave radiation, microwave-absorbing layers formed from appropriately thin metal films absorb a portion of the microwave energy and undergo resistive (ohmic) heating due to the electrical currents induced within the metal layer. Such absorbing metal layers are exceedingly thin and typically possess sheet resistances of 20-500 xcexa9/xe2x96xa1 (ohms per square of the materialxe2x80x94the ohms per square value can be obtained by cutting a square of any length on a side and measuring the resistance between two sides of the square with an ohm meter). It is impractical to measure the thicknesses of such films directly, and, therefore, their thicknesses are commonly specified in terms of optical density, which increases with metal thickness. For aluminum, sheet resistances of 20-500 xcexa9/xe2x96xa1 correspond to optical densities of approximately 0.10-0.70. The sheet resistance typically decreases as the optical density (i.e., thickness) increases.
Numerous susceptors are described in the prior art. Exemplary susceptors are disclosed in U.S. Pat. Nos. 5,530,231, 5,220,143, 5038,009, 4,914,266, 4,908,246, and 4,883,936, the disclosures of which are incorporated herein by reference.
Though conventional microwave susceptors are capable of heating, browning, or crisping microwave food products, the results of their use have not been entirely satisfactory. During use, conventional susceptors may undergo nonuniform heating when exposed to microwave radiation, causing some regions of a food product to be undercooked and other regions to be overcooked. Such non-uniform heating may result inherently from the susceptor itself, from microwave oven xe2x80x9chot spotsxe2x80x9d corresponding to regions of greater microwave intensity, or from non-uniform contact of the food product with the susceptor. In addition, conventional susceptors may overheat, become damaged, and cease to function as desired. Specifically, susceptor overheating is typically accompanied by shrinkage of the polymer layer or layers, leading to cracking (crazing) of the metallic layer. As a result, the susceptor may become less absorbing to microwave radiation and more transmitting, and the food product may, therefore, receive a greater amount of conventional dielectric heating from the microwave radiation than desired.
A number of approaches have emerged to address the above-mentioned problems. One of these involves the patterning of conventional metal microwave-absorbing layers by selective demetallization to control the amount of heating in predetermined regions of the susceptor. Another patterning approach entails disrupting rather than demetallizing microwave-absorbing layers in selected regions of susceptors. A number of techniques have been utilized to provide the desired patterning. Exemplary techniques are described in U.S. Pat. Nos. 5,614,259, 4,959,120, 4,685,997, 4,610,755, and 4,552,614, the disclosures of which are incorporated herein by reference.
Other approaches that address susceptor deficiencies utilize a separate shielding layer or device that substantially reflects and/or focuses microwave energy traveling from a microwave source before it reaches a microwave-absorbing susceptor layer. Metal layers of such shielding behavior have a relatively large thickness when compared with metallic susceptor layers formed from the same material by vacuum metallization techniques, hereafter also referred to as heavy-metal layers, typically possess sheet resistances of 1.0-5.0xcexa9/xe2x96xa1 and optical densities on the order of 1.0-2.5. As a result, such metal layers are relatively less absorbing than thinner metal layers and undergo substantially less heating when exposed to microwave radiation. Numerous shielding and/or intensifying structures are described in the prior art. Exemplary structures are disclosed in U.S. Pat. Nos. 5,300,746, 5,254,821, 5,185,506, and 4,927,991, the disclosures of which are incorporated herein by reference.
The use of heavy-metal microwave shields and focusing structures in conjunction with microwave-absorbing structures has been carried out with varying degrees of success and has been difficult to apply commercially. The benefits obtained by using such conventional structures are often offset by the increased complexity and expense of processing packaging materials with two or more metallic layers of different thicknesses. In an environment where packaging materials are disposable, minimizing complexity and cost while enhancing functionality is an important concern.
Accordingly, it is apparent that a significant need exists for simple, cost-effective microwaveable structures and formations that provide reliable, well-defined microwave heating, browning and/or crisping in predetermined regions and in predetermined amounts.
The present invention satisfies these and other objects by providing microwaveable formations comprising a heavy-metal layer (or layers) that is (are) selectively patterned to act as a microwave-absorbing layer, microwave shielding layer, and/or microwave focusing layer, all having the same thickness.
According to a first aspect of the present invention, a microwave laminate is provided comprising a first layer substantially transparent to microwave energy having an electrically insulating surface and at least one microwave-absorbing region of patterned electrically conducting film of substantially shielding thickness contiguous with the electrically insulating surface of the first layer. Each microwave-absorbing region is patterned to provide an increased effective electrical sheet resistance that allows the microwave-absorbing region to substantially absorb rather than reflect microwave energy. Thus a microwave susceptor is formed from an electrically conducting film that would ordinarily reflect a substantial portion of incident microwave energy if it were not patterned in a manner to absorb microwave energy.
The present invention further provides a package for microwave heating of food products comprising a first layer substantially transparent to microwave energy having a first surface disposed near or supporting an intended food product. At least one microwave-absorbing region of patterned electrically conducting film of substantially shielding thickness is disposed on at least the first surface of the first layer. Each microwave-absorbing region is patterned to provide an effective electrical sheet resistance that allows the microwave-absorbing region to substantially absorb rather than reflect microwave energy.
The present invention further satisfies the above-mentioned objectives, and others, by providing a microwave susceptor underlay comprising a heavy-metal film having a particular pattern and corresponding properties. The invention further provides a substantially non-absorbing microwave susceptor underlay comprising patterned regions of electrically conducting film of substantially shielding thickness disposed on a first layer substantially transparent to microwave energy having an electrically insulating surface. The microwave susceptor underlay may be positioned beneath a heavy-metal or conventional microwave susceptor or may be laminated to an electrically insulating surface of either type of microwave susceptor.